For a new sim outside of LFS, I'd jump on AC instantly the second online becomes available. Better to get things established quickly so you have a large base right off the bat. Obviously if the netcode is 1995 level then reevaluate after.
For your premier series, GTWS and LFSCART should be all you need. Each quarter of the year you can have a different series pop in like TBOC, STDC, LXC, SSC (Single seaters idk). Also once each quarter, at least one 'event' needs to be ran. This could be a large endurance race when GTWS is in hibernation mode. It can also be something off the wall like Autocross or Rallycross. Point is, don't get repetitive. KY250 and 500 are two big events for you right there which never change, and they should be the only ones that do not change.
For KY250 and 500, just make an insim which tracks how many times a person has "Major Damage Repair". They should only be able to do it two times, the third time it should exclude them from the race.